The present invention relates to a method of calibration of magnification of optical devices.
More particularly, it relates to a method of calibration of such optical devices as steppers for forming patterns on silicon wafers, for calibration of scanning electron microscopes, etc.
During the manufacture of ICs on silicon wafers, the steppers are utilized to produce devices on a wafer. If the stepper is not properly operating, the physical features of the device produced of silicon wafers can have non uniform dimensions and therefore they will not operate in the intended manner.
Methods for calibration of magnification of steppers for this application are not known, and it is believed that it is therefore advisable to provide such calibration. As for scanning electron microscopes, methods of calibration of magnification are as disclosed for example in U.S. Pat. Nos. 5,902,703; 4,618,938; and 5,825,043; however, they are not precise.
Accordingly, it is an object of present invention to provide a method of calibration of magnification of optical devices, which is a further improvement of the existing methods.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method of calibration of magnification of an optical device which includes providing a mask with a predetermined pattern; projecting radiation through the mask so as to form a pattern image; and comparing image of the pattern with a pattern on the mask to determine deviations of the image of the projected image from the patterns on the mask.
Where the method is designed in accordance with the present invention it eliminates the disadvantages of the prior art and provides for the highly advantageous results.
In accordance with a further feature of present invention the method includes comparing a size of the projected image with the size of the image on the mask.
In accordance with a further feature of present invention comparing the mask includes providing a mask which has at least two fields which having a plurality of features spaced from one another in one direction so that the directions of spacing of the features in said two fields are transverse to one another.
In accordance with a further feature of present invention the features in each of said field are evenly spaced from one another. In accordance with a further feature of present invention the features in each of said fields are parallel to one another.
In accordance with a further feature of present invention the method includes arranging the mask so that the features of at least one of said fields are aligned with a principal axis of the optical device.
In accordance with a further feature of present invention the features are arranged so that a spacing between the features of one of the fields is different from the spacing of the features of the other of said fields.
In accordance with a further feature of present invention the method includes moving an article on which the image is projected relative to the optical device so as to produce a plurality of images with a plurality of patterns, and the comparing includes comparing the plurality of patterns of the plurality of images with the pattern of the mask.
In accordance with a further feature of present invention the method includes projecting the pattern through the mask on the substrate, and obtaining the projected image on the substrate.
In accordance with a further feature of present invention the method includes passing the radiation through the mask to form patterns on a silicon wafer and obtaining an electronic image with a pattern, and comparing includes comparing the pattern of the electronic image from the silicon wafer with the pattern of the mask.
In accordance with a further feature of present invention the comparing includes comparing a deviation of a magnification of the pattern of the image relative to the pattern of the mask from a standard deviation.
In accordance with a further feature of present invention the method includes forming a mask by an optical source, and producing an interference pattern corresponding to the desired pattern of the mask.
In accordance with a further feature of present invention the method includes producing the mask by an optical source which generates an interference pattern and acts on a chemical substance causes etching of corresponding area of the mask so as to produce the desired pattern of the mask.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.